We measured the sensitivity of macaque ganglion cells to luminance and chromatic sinusoidal modulation. Phasic ganglion cells of the magnocellular pathway (M-pathway) were the more sensitive to luminance modulation, and tonic ganglion cells of the parvocellular pathway (P-pathway) were more sensitive to chromatic modulation. With decreasing retinal illuminance, phasic ganglion cells' temporal sensitivity to luminance modulation changed in a manner that paralleled psychophysical data. The same was true for tonic cells and chromatic modulation. Taken together, the data suggest strongly that the cells of the M-pathway form the physiological substrate for detection of luminance modulation and the cells of the P-pathway the substrate for detection of chromatic modulation. However, at high light levels, intrusion of a so-called luminance mechanism near 10 Hz in psychophysical detection of chromatic modulation is probably due to responses in the M-pathway, arising primarily from a nonlinearity of cone summation. Both phasic and tonic ganglion cells responded to frequencies higher than can be psychophysically detected. This suggests that central mechanisms, acting as low-pass filters, modify these cells' signals, though the corner frequency is lower for the P-pathway than for the M-pathway. For both cell types, the response phase at different frequencies was consistent with the cells' description as linear filters with a fixed time delay.

SUMMARY1. We measured the response of macaque ganglion cells to sinusoidally modulated red and green lights as the relative phase, 0, of the lights was varied.2. At low frequencies, red-green ganglion cells of the parvocellular (PC-) pathway with opponent inputs from middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones were minimally sensitive to luminance modulation (0 = 0 deg) and maximally sensitive to chromatic modulation (0 = 180 deg). V. C. SMITH AND OTHERS average, the weighting corresponded to the Judd modification of the luminosity function although there was considerable inter-cell variability.6. To account for results from MC-pathway cells, it was necessary to postulate a cone-opponent, chromatic input to their surrounds. We developed a receptive field model with linear summation of M-and L-cones to centre and surround, and with an additional M,L-cone opponent input to the surround. It proved possible to account for response phase and amplitude of both on-and off-centre cells. For both, the proposed cone-opponent input to the surround must consist of a (+ M -L) mechanism.7. The dependence of minimum response of MC-pathway cells on 0 closely resembles psychophysical results obtained using the same protocol, in which psychophysical modulation sensitivity was measured as a function of 0. The results provide strong corollary evidence linking the MC-pathway to psychophysical performance in heterochromatic flicker photometry.

Descriptions of receptive fields at subcortical levels of the visual system have mostly considered only the classical receptive field (CRF). A suppressive extraclassical receptive field (ECRF) has been demonstrated in relay cells within the primate lateral geniculate nucleus (LGN), but the quantitative properties and specific influence of the ECRF on the distinct magnocellular (MC), koniocellular (KC), and parvocellular (PC) pathways are not known. Here we quantified the effect of ECRF stimulation on visually responsive cells in the LGN of a diurnal New World primate, the marmoset. We show that for all cells, visually evoked responses are reduced by stimulation of the ECRF. The magnitude of the suppression is greatest for MC cells and smallest for PC cells. The effect of ECRF stimulation on KC cells is variable but always suppressive. We refer to these effects as extraclassical inhibition (ECI). The contrast-response relationship of the ECI parallels that of CRF-induced excitation for each cell class: for MC cells, ECI contrast threshold is close to 10% and the ECI saturates at 50% contrast, but the contrast dependence of ECI on PC cells is more linear. The ECI also contributes to contrast-dependent changes in spatial summation: on average for all LGN cells the radius of the excitatory spatial summation field (measured from aperture-tuning curves) at low contrast is 1.31 times that at high contrast. No consistent effects of orientation on ECI were seen. The data suggest that the suppressive component of the ECRF seen in cortical neurons could primarily be inherited from subcortical input streams.

SUMMARY1. We have studied the sensitivity of macaque retinal ganglion cells to sinusoidal flicker. Contrast thresholds were compared for stimuli which alternated only in luminance ('luminance flicker') or chromaticity ('chromatic flicker'), or which modulated only the middle-or long-wavelength-sensitive cones ('silent substitution').2. For luminance flicker, the lowest thresholds were those of phasic, non-opponent ganglion cells. Sensitivity was maximal near 10 Hz.3. Tonic, cone-opponent ganglion cells were relatively insensitive to luminance flicker, especially at low temporal frequencies, but were sensitive to chromatic flicker, thresholds changing little from 1 to 20 Hz. Those with antagonistic input from middle-and long-wavelength-sensitive (M-and L-) cones had a low threshold to chromatic flicker between red and green lights. Those with input from shortwavelength-sensitive (S-) cones had a low threshold to chromatic flicker between blue and green. Expressed in terms of cone contrast, the S-cone inputs to blue on-centre cells had higher thresholds than M-and L-cone inputs to other cell types.4. Phasic, non-opponent cells responded to high-contrast red-green chromatic flicker at twice the flicker frequency. This frequency-doubled response is due to a non-linearity of summation of M-and L-cone mechanisms. It was only apparent at cone contrasts which were above threshold for most tonic cells.5. M-or L-cones were stimulated selectively using silent substitution. Thresholds of M-and L-cone inputs to both red and green on-centre cells were similar. This implies that these cells' sensitivity to chromatic flicker is derived in equal measure from centre and surround. Thresholds of the isolated cone inputs could be used to predict sensitivity to chromatic flicker. The high threshold of these cells to achromatic contrast is thus, at least in part, due to mutual cancellation by opponent inputs rather than intrinsically low sensitivity.6. Thresholds of M-and L-cone inputs to phasic cells were similar at 10 Hz, and were comparable to those of tonic cells, suggesting that at 1400 td cone inputs to both cell groups are of similar strength.7. The modulation transfer function of phasic cells to luminance flicker was similar to the detection sensitivity curve of human observers who viewed the same stimulus. For chromatic flicker, at low temporal frequencies thresholds of tonic cells (red or B. B. LEE, P. R. MARTINAND A. VALBERG green on-centre cells in the case of red-green flicker or blue on-centre cells in the case of blue-green flicker) approached that of human observers. We propose the different cell types are the substrate of different channels which have been postulated on the basis of psychophysical experiments.8. At frequencies of chromatic flicker above 2 Hz, human sensitivity falls off steeply whereas tonic cell sensitivity remained the same or increased. This implies that high-frequency signals in the chromatic, tonic cell pathway are not available to the central pathway responsible for flicker detection.

Transfer of visual information from photoreceptors to ganglion cells within the retina is mediated by specialized groups of bipolar cells. At least 10 different morphological types of bipolar cells have been distinguished in Golgi studies of primate retina. In the present study, bipolar cell populations in the macaque monkey retina were identified by their differential immunoreactivity to a spectrum of antibody markers. This enabled their spatial density and photoreceptor connections to be analysed. An antibody against the beta isozyme of protein kinase C (PKCA beta) labelled many cone bipolar cells. Invaginating (presumed ON) cone bipolar cells and rod bipolar cells were preferentially labelled with a monoclonal antibody raised against rabbit olfactory bulb. Flat (presumed OFF) bipolar cells were labelled with an antiserum against the glutamate transporter protein (GLT-1). Different populations of diffuse cone bipolar cells, which contact 5-10 cones, could be distinguished. The GLT-1 antiserum preferentially labelled the flat diffuse bipolar cell type DB2 (Boycott and Wässle, 1991, Eur. J. Neurosci. 3:1069-1088) as well as flat midget bipolar cells. Antibodies to calbindin (CaBP D-28K) labelled the flat diffuse bipolar cell type DB3 and (possibly) the invaginating diffuse bipolar cell type DB5. An antibody against the alpha isozyme of PKC labelled an invaginating diffuse bipolar cell type (DB4) as well as rod bipolar cells. Comparison of the spatial density of cone bipolar cell populations with that of photoreceptors suggests that each bipolar cell class provides a complete coverage of the cone array (each cone is contacted by at least one member of every bipolar cell class). These results support the classification scheme of Boycott and Wässle (1991) by showing that different diffuse bipolar cell classes express different patterns of immunoreactivity, and they reinforce the view that different spatial and temporal components of the signal from the photoreceptor array are processed in parallel within the primate retina.

Colour vision in primates is mediated by cone opponent ganglion cells in the retina, whose axons project to the dorsal lateral geniculate nucleus in the visual thalamus. It has long been assumed that cone opponent ganglion cells project to the parvocellular layers of the geniculate. Here, we examine the role of a third subdivision of the geniculocortical pathway: the interlaminar or koniocellular geniculate relay cells. We made extracellular recordings in the dorsal lateral geniculate nucleus of the common marmoset Callithrix jacchus, a New World monkey in which the interlaminar cells are well segregated from the parvocellular layers. We found that one group of colour opponent cells, the blue-on cells, was largely segregated to the interlaminar zone. This segregation was common to dichromatic ('red-green colour-blind') and trichromatic marmosets. The result calls into question the traditional notion that all colour information passes through the parvocellular division of the retino-geniculo-cortical pathway in primates.

For those women with PND, psychological intervention is a better option than routine care, leading to clinically significant reduction of symptoms. Counselling may be as effective as group cognitive behaviour therapy. The benefits of psychological intervention may be maximized by being delivered on a one-to-one basis.

SUMMARY1. Heterochromatic flicker photometry is a way of measuring the spectral sensitivity of the human eye. Two lights of different colour are sinusoidally alternated at, typically, 10-20 Hz, and their relative intensities adjusted by the observer until the sensation of flicker is minimized. This technique has been used to define the human photopic luminosity, or VA, function on which photometry is based.2. We have studied the responses of macaque retinal ganglion cells using this stimulus paradigm. The responses of the phasic ganglion cells go through a minimum at relative radiances very similar to that predicted from the VA function. At this point, defined as equal luminance, an abrupt change in response phase was observed. A small residual response at twice the flicker frequency was apparent under some conditions.3. The spectral sensitivity of parafoveal phasic cells measured in this way corresponded very closely to that of human observers minimizing flicker on the same apparatus. 4. Minima in phasic cell activity were independent of flicker frequency, as is the case in the psychophysical task.5. The response minima of phasic cells obey the laws of additivity and transitivity which are important characteristics of heterochromatic flicker photometry.6. As the relative intensities of the lights were altered responses of tonic, spectrally opponent cells usually underwent a gradual phase change with vigorous responses at equal luminance. The responses of tonic cells treated individually or as a population could not be related to the VA function in any meaningful way.7. We conclude that the phasic, magnocellular cell system of the primate visual pathway underlies performance in the psychophysical task of heterochromatic flicker photometry. It is likely that other tasks in which spectral sensitivity conforms to the VA function also rely on this cell system.